Liquid crystal displays (LCD's) provide many advantages over conventional display devices, such as cathode ray tubes (CRT's). For example, LCD's have a very narrow profile, are relatively light, require less drive circuitry and have relatively low power consumption. Furthermore, from a safety standpoint, most LCD's eliminate the high voltage risk (up to several thousand volts) associated with CRT's.
There are several types of LCD's. However, the general concept of operation is the same for all types. A liquid crystal material is placed in a sealed, light transmissive chamber. Light transmissive electrodes are placed above and below the liquid crystal material. In one type of LCD utilizing what are called twisted nematic liquid crystals, when a sufficient electric potential is applied between the electrodes, the liquid crystal molecules change their alignment. The change in alignment alters the polarization state of light through the liquid crystal material. The chamber or cell essentially acts as a light shutter or valve. It lets either a maximum, minimum or some intermediate amount of light through.
By putting the liquid crystal chamber on top of a reflective backing, and locating a polarizer above the liquid crystal chamber, the top of the liquid crystal chamber will look either black (dim) or white (bright) depending on the alignment of the material (whether the valve is "closed" or "open"). By using a collection of electrodes, such things as letters, numbers, or graphics can be formed by applying appropriate electrical potential across certain electrodes which instruct corresponding liquid crystal areas if the chamber to pass or block light, to in turn form the appropriate visual pattern.
LCD's can be generally divided into two categories: segmented LCD's and grid-matrix LCD's. Segmented LCD's are LCD's in which the electrodes are constructed in certain predetermined shapes of segments. Each segment can be used alone or in combination with other segments to form the desired visual pattern.
A common type of segmented LCD is the so-called "seven segment display." As the name suggests, the "seven segment display" comprises seven elongated rectangular segments arranged into a template or pattern which can form any numeral. Each segment is controlled to simply turn "on" or "off"; that is, transmit maximum light (appears brighter or white) or transmit no light (appears dim or black). The appropriate segments are turned "black" to form the desired numeral if a "normally white" background is used. It is to be understood that depending on the design choice, the background can be either normally black or normally white.
The advantage of a segmented display is the simplicity of the display and associated structure. Only a few segments must be operated to form each visual display pattern. However, the possible visual display patterns available are limited to patterns which incorporate the predetermined segment shapes.
Grid-matrix LCD's can be used to form more complex visual display patterns. Grid-matrix LCD's comprise a large number of very small independent electrodes positioned in a grid pattern in a plane in the liquid crystal material chamber. Each of the independent electrodes typically creates a picture element or pixel. Pixels are generally configured as small square segments arranged in rows and columns forming a matrix. Corresponding numbers of column and row electrodes are correlated with the rows and columns of pixels. An electric potential can therefore be applied to any pixel by the selection of appropriate row and column electrodes and a desired visual display pattern can be generated.
The individual pixels comprise dots or small portions of the overall picture or graphic to be displayed. A graphic control device therefore produces the appropriate instructions to "drive" the matrix of pixels to appropriately reconstruct the image desired. In other word, the control circuitry must send appropriate voltages to appropriate electrodes at appropriate times to form the desired image.
One disadvantage of grid-matrix LCD's is that due to the large number of pixels required to form most visual display patterns, grid-matrix LCD's can be cost prohibitive. Typically, all of the pixels in each row and column in a conventional rectangular grid-matrix LCD are connected to expensive drive electronics so that each and every pixel is operable for producing a visual display pattern. Thus, in order to produce a conventional grid-matrix LCD with a resolution of 20.times.40, 800 pixels must be evenly spaced across the LCD with each pixel connected to expensive drive electronics.
Graphical LCD's, such as conventional grid-matrix LCD's, are normally multiplexed. As the number of pixels in the display increases, the multiplex ratio (the inverse of the amount of time that an individual pixel is turned on) also increases. As the multiplex ratio of an LCD increases, the contrast ratio and readability of the display goes down. Thus, another disadvantage of conventional grid-matrix LCD's is that the multiplex ratio is relatively high and thus the contrast and readability of a conventional grid-matrix LCD is relatively low.
As a consequence of the large number of pixels and associated drive electronics, conventional grid-matrix LCD's are relatively more expensive than simpler segmented LCD's. Furthermore, the contrast and readability of conventional grid-matrix LCD's is relatively lower than simpler segmented LCD's. Thus, there is a need for a cost effective, high contrast, high readability, grid-matrix LCD which can be used to produce relatively complex visual display patterns.